1. Technical Field
The present invention relates to a semiconductor substrate, a semiconductor device, and a semiconductor substrate manufacturing method.
2. Related Art
A nitride compound semiconductor has greater dielectric breakdown voltage and greater bandgap energy than a silicon semiconductor, and is therefore expected to be used as material in elements with high withstand voltage. As a device that uses a nitride compound semiconductor, an AlGaN/GaN HFET is known that achieves higher withstand voltage by doping the nitride compound semiconductor with carbon, as shown in Patent Document 1, for example.
The following are related prior art documents.    Patent Document 1: Japanese Patent Application Publication No. 2007-251144    Non-Patent Document 1: J. E. Northrup, “Screw dislocations in GaN: The Ga-filled core model”, Appl. Phys. Lett., American Institute of Physics, 2001, Vol. 78, Issue 16, p. 2288 Non-Patent Document 2: Debdeep Jana, et al., “Effect of scattering by strain fields surrounding edge dislocations on electron transport in two-dimensional electron gases”, Appl. Phys. Lett., American Institute of Physics, 2002, Vol. 80, Issue 1, p. 64
A gallium nitride (GaN)-based semiconductor is epitaxially grown on a silicon substrate with a surface orientation of (111), but the difference between the lattice constant of the GaN and the silicon (111) surface is approximately 17%, which is quite large. Therefore, the dislocation density of the grown GaN exceeds 1010 cm−2. Screw dislocation and edge dislocation are types of dislocation occurring in the GaN. When the screw dislocation density increases, the leak current of the transistor using GaN increases, as shown in Non-Patent Document 1, for example. When the leak current increases, the withstand voltage of the transistor cannot be increased. When the edge dislocation density increases, the mobility in the transistor using GaN is decreased, as shown in Non-Patent Document 2, for example.
Even when the withstand voltage is increased by doping the nitride compound semiconductor with carbon, the leak current caused by spiral dislocations cannot be decreased. Furthermore, the drop in mobility caused by edge dislocations cannot be eliminated.